Comb generators are a common manner of producing a series of frequency harmonics known as a “comb” from a single input signal. The output spectrum of harmonics can then be used in various applications, software or hardware into which the comb generator can be incorporated such as in frequency synthesizers, frequency multipliers, test sets, or the like, or can be combined with other elements such as bandpass filters, amplifiers and the like to form various kinds of equipment.
Common comb generator designs incorporate step recovery diodes (SRDs) that are used to sharpen an edge of an input signal, which, in accordance with Fourier principals, results in an increase in the scope and magnitude of the harmonics across the frequency spectrum. In other words, the sharp time domain transition associated with the sharpened edge results in increased energy across the frequency spectrum at frequencies corresponding to at least one set of harmonics associated with the input signal frequency such as the set of odd or the set of even harmonics of the input signal frequency.
Limitations arise however with SRD-based comb generators. For example, as harmonics frequencies increase, SRD efficiency drops monotonically. Accordingly, most of the harmonic energy is wasted in the lower frequency bands that are often undesired for practical use. In microwave applications, for example, a comb generator is chose based on the ability to generate output tones in higher harmonic frequency bands so as to minimize the number of frequency multipliers required in the system. As will be appreciated by those skilled in the art, the spectrum output level associated with an SRD based generator roughly follows a 1/n drop. Further a typical SRD based generator does not operate well at frequencies above 20 GHz and often experiences instability with performance repeatability issues. Extensive tuning and re-calibration is often required with SRD based generators to achieve acceptable performance.
Other means for generating harmonics exist such as through the use of fiber optic or photonic processing of an input pulse in a series of optical components. Problems with the fiber optic approach however include the expense and complicated nature of the equipment, excess power consumption and the relative unsuitability of the approach for use in a small scale integrated application.
In general some prior art approaches exist to generate sharp edges, however these approaches include simply shaping of existing pulse trains, use of DC level shifts, and the like. Other approaches involve the use of a stub to generate a reflected signal for edge sharpening. However such approaches disrupt the transmission line impedance properties of the system and can cause circuit matching problems and other anomalies over wider portions of the frequency band.
It would therefore be desirable for a comb generator having a high efficiency and high performance in high frequency applications. Such a comb generator could preferably provide a flat output spectrum in the desired harmonic frequency band and have little or no wasted energy in lower frequency harmonic bands.